All this discussion of multifamily living reminds me of one of the things that struck me most strongly about reading fiction set in the early part of the twentieth century: the commonness of having boarders (usually single men) renting rooms in family homes. This seemed most typical for older women to use for income in lieu of outside jobs, but there's no reason it would have to be this way.
Renting rooms in what would otherwise be single-family homes is foreign to our modern sensibilities (and probably now violates many zoning ordinances), but how many McMansion owners could have swung their ARM payments by doubling up the kids and renting spare bedrooms to young students or professionals?